Study On Ytterbium-doped Fiber Laser Based On Carbon Quantum Dots Saturable Absorber

In the past few decades,pulsed fiber laser has had unique application value in optical communication,biomedicine,mechanical precision machining and other major fields due to its high peak power and narrow pulse width.Passive Q-switched and mode-locked technology using a saturable absorber(SA)are one of the effective methods to obtain pulsed laser,which has the advantages of high cost-effectiveness,low optical loss,and easy integration.Common SA materials include semiconductor saturable absorption mirrors(SESAMs),organic dyes and low-dimensional materials(such as carbon nanotubes,graphene,black phosphorus,transition metal sulfides and topological insulators).They have excellent electrical and optical properties,including quantum Hall effect,high carrier mobility and nonlinear optical effect.However,these SA materials also have shortcomings,such as high insertion loss,low modulation depth,low saturation depth,low damage threshold,and poor environmental stability.Thus,it is of great significance to explore saturable absorber materials with excellent nonlinear optical parameters,good stability,low cost,and high damage threshold.In this paper,carbon quantum dots(C-dots)were prepared by using a zeolite molecular sieve with a uniform pore structure as a template.The experimental results show that C-dots have good nonlinear saturable absorption properties.The prepared C-dots can be used as a saturable absorber to realize the output of 1?m-band pulsed laser,which is a low-dimensional material with commercial application prospects.The main work of this paper is as follows:1.Firstly,the development and application of fiber lasers are introduced.Starting from the pulse generation mechanism,the Q-switching and mode-locking technologies are analyzed,and the passive Q-switching and mode-locking principles based on saturable absorbers are emphatically introduced,which provides a theoretical basis for the experiment of all-fiber pulse fiber lasers.The development and classification of low dimensional materials saturable absorbers are discussed,and the research status of low dimensional materials saturable absorbers is introduced in detail.2.C-dots were prepared using a zeolite molecular sieve as a template.SAPO-46(AFS)zeolite molecular sieve was prepared by hydrothermal method with crystallization time of 40 h,the crystallization temperature of 180°C,dipropylamine(DPA)as a template,molar ratio of gel component of 0.6 Si O₂:1.0 Al₂O₃:1.2 P₂O₅:3.1 DPA:600 H₂O:1.6 HF.SEM results show that the AFS crystal has hexagonal prism morphology with an average size of 100?m.Using organic amine DPA as a carbon source,C-dots were obtained by carbonization of AFS crystal and removal of AFS crystal template.TEM and Raman spectra showed that the size distribution of C-dots was in the range of 1.5 nm-4 nm,and the lattice spacing was 0.21 nm.C-dots contained graphene-like structures.By using the balanced dual detector system,it is found that C-dots have saturable absorption characteristics.The modulation depth of C-dots in the 1?m band is 21.8%,and the saturated absorption intensity is4.7 MW/cm~2.3.The 1?m-band pulse laser output is realized by using C-dots as a saturable absorber.In the pump power range of 78 m W-181 m W,the stable mode-locked pulse output is obtained.The central wavelength of the mode-locked pulse output spectrum is 1043.9 nm,the pulse width is 715 ps,the repetition rate is 19.6 MHz,the corresponding pulse period is 51 ns,and the signal-to-noise ratio is 73 d B.The laser pumping slope efficiency is 5.5%.In the same resonant cavity,the Q-switched pulse can also be obtained by adjusting the pump power and polarization controller.In the pump power range of 60 m W-120 m W,the repetition rate of Q-switched pulse is 37.42 k Hz-56.87 k Hz.When the pump power is 80 m W,the central wavelength of the Q-switched pulse output spectrum is 1035.8 nm,and the corresponding pulse duration is 2.2?s.The pumping slope efficiency of the laser is1.2%.Zeolite-based C-dots is an excellent SA materials.